A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or antiparallel to this direction.
In the lithographic apparatus, various parts are fastened to one another using a fastener. It is known, for example, to use bolt or screw fasteners for fastening apparatus parts. For example, a first apparatus part, such as an optical component, may be clamped onto a second apparatus part, such as a base, using one or more such fasteners.
The known bolt or screw fastener includes a shaft having screw thread of a certain pitch, as well as a driving head for driving the bolt or screw into a suitable aperture of a respective apparatus part. After assembly, the bolt or screw clamps the apparatus parts together, using cooperation of the screw thread of the shaft and the screw aperture, and the driving head.
Particularly, in the known apparatus, the fastener can be used for locking an apparatus component in a desired position onto a base part of the apparatus. However, the known fastener tends to displace the apparatus component during assembly, when the fastener is being driven to a locking position. Such a displacement may involve, for example, a rotation and/or translation. After assembly, the fastener may induce internal stresses in the component which is locked into position by the fastener. Such stresses may result in unwanted positional and rotational drift of the component. Such drift of apparatus components may hamper a desired operating accuracy of the apparatus, making it difficult, if not impossible, to manufacture devices with desired small features using a lithographic process.
Further, a certain torque is applied during assembly to create a certain clamping force between the apparatus component and the base part. This torque is induced into the apparatus component via the screwhead pivot face (and thread windings engaged of the fastener), causing deformations in weak sections of the apparatus component as well as internal material stress. This is a second potential source of undesired positional and/or rotational drift of the apparatus component after assembly.